River Salinity in Coastal Bangladesh in a Changing Climate

Submitted by Susmita Dasgupta On Mon, 03/31/2014

With a virtual certainty that sea-level rise (SLR) will continue beyond 2100 even if greenhouse gas emissions were stabilized today, it is essential that we gain understanding of the potential impacts of SLR and begin planning adaptation, especially for countries with major risk of SLR. The urgency of responding to the growing alarm over climate change effects worldwide is hitting headlines this week, as the Intergovernmental Panel on Climate Change (IPCC) just released its Climate Change 2014[1] report warning that climate change is already having widespread effects on every continent and throughout the world’s oceans.

Most research to date has focused on the long-run effects of progressive inundation from SLR, along with associated losses from heightened cyclone-induced surges. Increased salinity from saltwater intrusion, along with its impact on livelihoods and adaptation alternatives, have not been investigated in great detail despite the expectation that saltwater intrusion will worsen in low-lying coastal areas around the world due to the changing climate.

Recently, to look more closely at the impacts of climate change on salt water intrusion, I joined a group of experts from the Institute of Water Modeling in Bangladesh and Professor Ainun Nishat, Vice Chancellor of BRAC University in conducting an analysis in coastal Bangladesh[2]. Bangladesh is one of the most vulnerable countries in the world to SLR, with two-thirds of its land area less than 5 meter above sea level. Sporadic measurement of soil and water salinity along the coastal area indicates intrusion of saline waterfront toward land already occurring in many areas. As the southwest coastal region of Bangladesh already has a serious problem from the spread and increasing intensity of salinization of river water, we focused our study of the impacts of climate change on that region.

Tidal waves from the Indian Ocean travel through the deeper part of the Bay of Bengal and approach the coast of Bangladesh from the south, where the turbulence generated by both wind and tidal currents mixes this saline water with the fresh water from the upstream rivers. Thus climate-induced changes in sea levels, temperature, rainfall, and in riverine flows from the Himalayas all affect river water, so they were included in our analysis by integrating information on climate change, hydrological models, and geographic overlays. We assessed the vulnerability of the southwest coastal region of Bangladesh to salt water intrusion by 2050, using a number of alternative scenarios for climate change and for upstream withdrawal of fresh water, also taking into account the projected land subsidence of the Ganges Delta.

Overall, our findings indicate that climate change will cause significant changes in river salinity in the southwest coastal area of Bangladesh during dry season (October to May) by 2050, which will likely lead to significant shortages of drinking water in the coastal urban areas, scarcity of water for irrigation for dry-season agriculture and significant changes in the coastal aquatic ecosystems. In addition, increase in salinity is expected to induce an overall shift in the World Heritage Sunderbans mangrove forest from Sundari (the single most dominant and important species) to Gewa and Guran.

Our estimates further indicate that a number of districts will be most adversely affected by the increase in river salinity in a changing climate. In particular, we find even in the best future scenario (least change in salinity from the baseline, March 2012), fresh water (0-1ppt) river zones will be lost entirely in Barguna, Jhalokoti, Khulna and Patuakhali districts. In the worst scenario (most change from the baseline, March 2012), over 90 percent of fresh river water in Pirojpur, Bagerhat and in Barisal will be at risk. Under this worst scenario, river water (with salinity < 2ppt) will no longer be utilizable for dry season agriculture in Barguna, Bhola, Jhalokati, Khulna, and Patuakhali districts. Even in the best future scenario, fresh water scarcity will be severe: for example, 98 percent of rivers in Khulna and 97 percent of rivers in Bhola will be adversely affected.

Changes in river salinity and the availability of freshwater also will affect the productivity of a number of capture fisheries, along with adverse effects in the wild habitats of fresh water fish and giant freshwater prawn. One serious potential effect of the probable decline in biodiversity of freshwater low value wild fish species from increased river salinity is on nutrition of the rural poor, since such fish serve as an important source of dietary protein for rural regions of Bangladesh where there is a high incidence of poverty. On the other hand, the changes may enhance opportunities for brackish water aquaculture, for example, Black Tiger Shrimp and Tilapia farming in coastal districts. This may offer increased livelihood opportunities for the rural poor in the future. Assessments of such location-specific economic impacts of the changes in river salinity, and identification of suitable adaptation alternatives, are urgent and warrant further analysis.